1. Field of the Invention
This invention relates to a buried type semiconductor laser device, which effectively suppresses ineffective current that is useless for laser oscillation even when current injected into the device is increased.
2. Description of the Prior Art
Buried type semiconductor laser devices, in which an active layer for laser oscillation is surrounded by semiconductor layers having a refractive index smaller than that of the active layer and an energy gap larger than that of the active layer, are, advantageous in that laser oscillation can be attained in a stable transverse mode at a low threshold current level and modulation can be attained at high speed, and accordingly they have been used as light sources for optical communication systems and/or optical measuring systems using optical fibers. For these reasons, they are industrially important devices. However, with such buried type semiconductor laser devices, ineffective current not passing through the active layer greatly increases with an increase in current injected into the devices, which causes limitations on the maximum value of the output power of the devices. Moreover, the ineffective current increases with a rise in temperature, which causes limitations on the temperature ranges in which the laser devices are used and which causes difficulties in the practical application of these buried type semiconductor laser devices, especially InGaAsP/InP semiconductor laser devices having a light-emitting wavelength in the range of 1.1 to 1.6 .mu.m at which optical fibers undergo little optical loss.
The reason why the above-mentioned ineffective current arises seems to be as follows: Buried type semiconductor laser devices are, for example, provided with the structures shown in FIGS. 14 and 15. The laser device shown in FIG. 14, is produced as follows: On an n-InP substrate 1, an n-InP buffer layer 2, a non-doped InGaAsP active layer 3, and a p-InP cladding layer 4 are successively grown by an epitaxial growth technique. The resulting multi-layered epitaxial growth crystal is subjected to a chemical etching treatment to form a mesa. Then, on both sides of the mesa, a p-InP burying layer 5 and an n-InP burying layer 6 are grown. The laser device shown in FIG. 15 is produced as follows: On an n-InP substrate 1, a p-InP burying layer 5 and an n-InP burying layer 6 are successively grown by an epitaxial growth technique. The resulting epitaxial growth crystal is subjected to a chemical etching treatment to form a channel. Then, an n-InP buffer layer 2, an InGaAsP active layer 3, and a p-InP cladding layer 4 are successively grown in the channel.
The device produced according to the production mode shown in each of FIGS. 14 and 15 attains laser oscillation depending upon the injected current 7 passing through the active layer 3. Since the p-n junction at the interface between the burying layers 5 and 6 positioned at the sides of the active layer 3 is reversely biased, little current passes through the burying layers 5 and 6 when the injected current 7 is small. However, a considerable amount of current passes through the burying layers 5 and 6 positioned at the sides of the active layer 3 as the injected current 7 increases. This is because a thyristor composed of the cladding layer 4, the n-burying layer 6, the p-burying layer 5 and the buffer layer 2 (or the substrate 1) is made conductive by a gate current 7b which flows from the cladding layer 4 to the burying layer 5 (Higuchi et al: Laser Kenkyu Vol. 13, p. 156, 1985). If the active layer 3 is formed at the interface between the lower burying layer 5 and the upper burying layer 6, the injected current (i.e., the gate current) 7b will be reduced. However, such a precise control of the thickness of layers cannot be made using liquid phase epitaxy and chemical etching techniques at the present. Thus, the ineffective current mentioned above cannot be prevented.